Hi
I have proffesional Medical forum and i want to allow spesific users to register.
what can i do?
can i add "registion code" to the registering page?
can i enter specific email adresses that can register? (s.e 500 adresses)
anoher solution?

Think I should have made that a bit clearer.
I did not mean on it's own, I meant in addition to the Q&A and other measures used to prevent Spam registrations.
I use the 'Sortablescaptcha' from Derky as well as the Account activation by Admin on a moderately large busy site and only get a few activation requests.
The most I have seen so far is three and often there is none on any particular day.

The OP mentioned only allowing from certain email addresses and the Account activation by Admin shows those so thought it would be a useful addition for the OP.